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SETUID(2) Linux Programmer's Manual SETUID(2)


     setuid - set user identity


     #include <sys/types.h>
     #include <unistd.h>
     int setuid(uid_t uid);


     setuid()  sets  the  effective  user ID of the calling process.  If the
     calling process is privileged (more precisely: if the process  has  the
     CAP_SETUID  capability  in  its user namespace), the real UID and saved
     set-user-ID are also set.
     Under Linux, setuid() is implemented like the POSIX  version  with  the
     _POSIX_SAVED_IDS  feature.  This allows a set-user-ID (other than root)
     program to drop all of its user privileges, do some un-privileged work,
     and then reengage the original effective user ID in a secure manner.
     If  the  user  is root or the program is set-user-ID-root, special care
     must be taken: setuid() checks the effective user ID of the caller  and
     if  it  is the superuser, all process-related user ID's are set to uid.
     After this has occurred, it is impossible for  the  program  to  regain
     root privileges.
     Thus, a set-user-ID-root program wishing to temporarily drop root priv-
     ileges, assume the identity of an unprivileged user,  and  then  regain
     root privileges afterward cannot use setuid().  You can accomplish this
     with seteuid(2).


     On success, zero is returned.  On error, -1 is returned, and  errno  is
     set appropriately.
     Note:  there  are cases where setuid() can fail even when the caller is
     UID 0; it is a grave security error to  omit  checking  for  a  failure
     return from setuid().


     EAGAIN The  call would change the caller's real UID (i.e., uid does not
            match the caller's real UID), but there was a temporary  failure
            allocating the necessary kernel data structures.
     EAGAIN uid  does not match the real user ID of the caller and this call
            would bring the number of processes belonging to the  real  user
            ID  uid  over  the  caller's RLIMIT_NPROC resource limit.  Since
            Linux 3.1, this error case no longer occurs (but robust applica-
            tions  should  check  for  this  error);  see the description of
            EAGAIN in execve(2).
     EINVAL The user ID specified in uid is not valid in  this  user  names-
     EPERM  The  user is not privileged (Linux: does not have the CAP_SETUID
            capability) and uid does not match the real UID  or  saved  set-
            user-ID of the calling process.


     POSIX.1-2001, POSIX.1-2008, SVr4.  Not quite compatible with the 4.4BSD
     call, which sets all of the real, saved, and effective user IDs.


     Linux has the concept of the filesystem user ID, normally equal to  the
     effective  user ID.  The setuid() call also sets the filesystem user ID
     of the calling process.  See setfsuid(2).
     If uid is different from the old effective UID,  the  process  will  be
     forbidden from leaving core dumps.
     The original Linux setuid() system call supported only 16-bit user IDs.
     Subsequently, Linux 2.4 added setuid32() supporting  32-bit  IDs.   The
     glibc  setuid() wrapper function transparently deals with the variation
     across kernel versions.
 C library/kernel differences
     At the kernel level, user IDs and group IDs are a per-thread attribute.
     However,  POSIX  requires  that all threads in a process share the same
     credentials.  The  NPTL  threading  implementation  handles  the  POSIX
     requirements  by  providing  wrapper  functions  for the various system
     calls that change process  UIDs  and  GIDs.   These  wrapper  functions
     (including  the  one  for  setuid()) employ a signal-based technique to
     ensure that when one thread  changes  credentials,  all  of  the  other
     threads in the process also change their credentials.  For details, see


     getuid(2), seteuid(2), setfsuid(2), setreuid(2), capabilities(7),  cre-
     dentials(7), user_namespaces(7)


     This  page  is  part of release 4.16 of the Linux man-pages project.  A
     description of the project, information about reporting bugs,  and  the
     latest     version     of     this    page,    can    be    found    at

Linux 2017-09-15 SETUID(2)

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